Solder fatigue acceleration prediction and testing results for different thermal test- and field cycling environments

Abstract

Due to the relatively short use time of lead-free solders, the issue of solder joint service life is not fully resolved yet. In particular, there is a lack of testing data for long term thermal cyclic under benign cyclic conditions as well as a on a reliable predictive model for solder fatigue acceleration. This subject has been addressed by long-term testing. Boards from series production were subjected to field cycles 23°C/93°C, 6 hours. After 3 ½ and 4 ½ years or 4800 and 6500 cycles, respectively, the test boards were analyzed. Both solders under investigation, SAC 305 and Innolot, were additionally tested under -40/150°C, 1hour, test cycling conditions. An acceleration factor of approximately 11 was figured out based on computer tomography and cross sectioning analyses. For SAC acceleration predictions based on different published analytical models (Norris/Landsberg equations) were compared to the testing results. The predicted range of acceleration factors was 4.9-39.4, i.e. this type of prediction can be misleading. A second type of comparison was made based on finite element analysis and a related phenomenological model, which gave predictions of the acceleration at least on the save side. For Innolot less fatigue was generally seen, however, kinds of brittle cracking were observed different for field and test loadings. In a final part of the paper an additional study on the effect of two test cycles, 0/100°C and -40/125°C, 1 hour, on multi row QFN fatigue is discussed which shows that involvement of plastic package with Tg in the cyclic range can cause acceleration totally different from analytical predictions.